bone mineral content; this then allows the calculation of BMD expressed in absolute
terms of grams of mineral per square centimeter scanned (g/cm^2 ). DEXA can
measure central (hip and spine) and peripheral (forearm) sites, and also has the
capacity to be used to perform a full body scan. It remains the only available
technique that can confirm osteoporosis diagnosis (Schousboe et al. 2013 ).
For postmenopausal women and men aged 50 years and older, the World Health
Organization (WHO) has established diagnostic T-score criteria (normal, low bone
mass, and osteoporosis) based on measurements at the spine, hip, or forearm.
T-scores compare an individual’s BMD to that of the average healthy 30-year-old
of the same sex and when possible, ethnicity. Broadly, ethnicity is a proxy for a
variety of potentially shared cultural, religious, dietary, geographic, and genetic
factors that affect bone mass, and for this reason, the interplay between ethnic
background and sex is a more valuable determinant of osteoporosis risk than sex
alone (Cauley 2011 ; Nelson and Villa 1999 ). Measured in standard deviation units
(SD), osteoporosis is identified when a T-score is−2.5 SD while clinically low
bone mass is confirmed at−1to−2.5 SD (ISCD 2013). Z-scores or“age-matched”
values may also be informative as they compare individual BMD with the average
BMD for their age group, while accounting for sex and ethnicity. In younger
populations, T-scores and Z-scores are usually similar; however, among older
individuals, lower bone density values are more prevalent, thereby shifting the
mean for these age cohorts. Age-matched comparisons can therefore be misleading
because even if individual bone density falls within the average Z-score range,
BMD may still be clinically low.
Although considered the“gold standard”in measuring bone density, DEXA
scanners are expensive, not portable, and expose participants to ionizing radiation;
these factors limit their use for non-clinical and screening purposes. Unfortunately,
DEXA as a diagnostic tool is not universally available in many countries leading
bone specialists to employ alternative technologies to predict risk of osteoporosis
and fracture.
Quantitative ultrasound (QUS)techniques offer an alternative means of mea-
suring skeletal health and are useful for screening individuals for osteoporosis risk.
QUS provides information on both cortical and trabecular bone, and its measure-
ments are influenced by several factors, including bone micro-architecture (Bartl
and Frisch 2004 ). This technology has demonstrated utility both in research and in
population-level screening, in part because QUS devices are portable and relatively
inexpensive (Madimenos et al. 2014 ; Stieglitz et al. 2015 ). Moreover, validated
calcaneal ultrasounds are highly correlated with DEXA and may predict fractures in
postmenopausal women and in men 65 years and older (Langton and Langton
2000 ). While there is certainly variation in bone density among different skeletal
regions, the calcaneus is a weight-bearing bone, similar to the femur, and therefore
is an ideal single-site measure of bone density (Barkmann et al. 2000 ; Gerdhem
et al. 2008 ; Nayak et al. 2006 ).
More recently, multiskeletal site ultrasonometers have gained popularity and a
few studies have demonstrated their use in clinical and field-based settings
(Barkmann et al. 2000 ). These devices typically operate using a hand-held probe to
12 Bone Health in Midlife Women 261